U.S. patent number 5,169,812 [Application Number 07/760,294] was granted by the patent office on 1992-12-08 for catalyst and process for producing aromatic compounds from c.sub.2 -c.sub.6.
This patent grant is currently assigned to UOP. Invention is credited to Christopher D. Gosling, Tamotsu Imai, Joseph A. Kocal, Paul J. Kuchar.
United States Patent |
5,169,812 |
Kocal , et al. |
December 8, 1992 |
Catalyst and process for producing aromatic compounds from C.sub.2
-C.sub.6
Abstract
A catalyst for converting C.sub.2 to C.sub.6 aliphatic
hydrocarbons to aromatics is described. The catalyst contains a
zeolite, an aluminum phosphate binder and a gallium component.
Examples of zeolites which can be used are the ZSM family of
zeolites, with ZSM-5 being a specific example. The catalyst is
characterized in that it is tolerant to exposure to hydrogen at
tempertures of about 500.degree. to about 700.degree. C. The
catalyst's tolerance to hydrogen exposure is the result of treating
the catalyst with an aqueous solution of a weakly acidic ammonium
salt or a dilute acid solution at a temperature of about 50.degree.
to about 100.degree. C. for a time of about 1 to about 48 hours,
followed by calcination. A process for preparing the catalyst is
also described.
Inventors: |
Kocal; Joseph A. (Gurnee,
IL), Imai; Tamotsu (Mt. Prospect, IL), Kuchar; Paul
J. (Hinsdale, IL), Gosling; Christopher D. (Roselle,
IL) |
Assignee: |
UOP (Des Plaines, IL)
|
Family
ID: |
25058667 |
Appl.
No.: |
07/760,294 |
Filed: |
September 16, 1991 |
Current U.S.
Class: |
502/61; 502/71;
502/85; 502/86 |
Current CPC
Class: |
B01J
29/40 (20130101); C07C 2/00 (20130101); C07C
2/00 (20130101); C07C 15/00 (20130101); B01J
2229/26 (20130101); B01J 2229/36 (20130101); B01J
2229/42 (20130101); C07C 2527/167 (20130101); C07C
2529/04 (20130101) |
Current International
Class: |
B01J
29/00 (20060101); B01J 29/40 (20060101); C07C
2/00 (20060101); B01J 000/00 () |
Field of
Search: |
;502/61,85,86,71 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dees; Carl F.
Attorney, Agent or Firm: McBride; Thomas K. Snyder; Eugene
I. Molinaro; Frank S.
Claims
We claim as our invention:
1. A catalyst for converting C.sub.2 to C.sub.6 aliphatic
hydrocarbons to aromatics comprising a zeolite having a Si:Al ratio
greater than about 10 and a pore diameter of about 5-6 .ANG., a
gallium component and an aluminum phosphate binder, the catalyst
characterized in that it is tolerant to exposure to hydrogen at a
temperature of about 500.degree. to about 700.degree. C.
2. The catalyst of claim 1 where the zeolite is a ZSM family
zeolite.
3. The catalyst of claim 1 where the zeolite concentration ranges
from about 30 to about 90 weight percent of the catalyst.
4. The catalyst of claim 1 where the gallium content on the
catalyst varies from about 0.1 to about 5 weight percent, as the
metal, of the catalyst.
5. A process for preparing a catalyst for converting C.sub.2 to
C.sub.6 aliphatic hydrocarbons to aromatics comprising: a) forming
particles from a mixture of a zeolite and an aluminum phosphate
binder; b) calcining said particles at a temperature of about
500.degree. to about 700.degree. C. and for a time of about 1 to
about 15 hours; c) impregnating the calcined particles with a
gallium salt; d) heating the gallium containing particles in
hydrogen at a temperature of about 500.degree. to about 700.degree.
C. for a time of about 1 to about 15 hours, followed by heating in
an air/steam mixture at a temperature of about 400.degree. to about
700.degree. C. for a time of about 1 to about 10 hours; e) treating
the particles of step (d) with an aqueous solution of a weakly
acidic ammonium salt or a dilute acid solution at a temperature of
about 50.degree. to about 100.degree. C. for a time of about 1 to
about 48 hours; and f) calcining the particles at a temperature of
about 500.degree. to about 700.degree. C. for a time of about 1 to
about 15 hours, thereby providing said catalyst.
6. The process of claim 5 where the gallium content on the catalyst
varies from about 0.1 to about 5 weight percent, as the metal, of
the catalyst.
7. The process of claim 5 where the zeolite concentration varies
from about 30 to about 90 weight percent of the catalyst.
8. The process of claim 7 where the zeolite concentration varies
from about 50 to about 70 weight percent of the catalyst.
9. The process of claim 5 where the zeolite is a ZSM family
zeolite.
Description
FIELD OF THE INVENTION
The present invention relates to a catalyst and to a process using
the catalyst for the production of aromatic hydrocarbons via the
dehydrocyclodimerization of C.sub.2 -C.sub.6 aliphatic
hydrocarbons. The catalyst is characterized in that it is tolerant
to exposure to hydrogen at a temperature of about 500.degree. to
about 700.degree. C.
BACKGROUND OF THE INVENTION
Dehydrocyclodimerization is a process in which aliphatic
hydrocarbons containing from 2 to 6 carbon atoms per molecule are
reacted over a catalyst to produce a high yield of aromatics and
hydrogen, with a light ends byproduct, C.sub.2 -C.sub.4 recycle
product and a trace C.sub.4.sup.+ nonaromatic byproduct. This
process is well known and is described in detail in U.S. Pat. Nos.
4,654,455 and 4,746,763 which are incorporated by reference.
Typically, the dehydrocyclodimerization reaction is carried out at
temperatures in excess of 500.degree. C., using dual functional
catalysts containing acidic and dehydrogenation components. The
acidic function is usually provided by a zeolite which promotes the
oligomerization and aromatization reactions, while a non-noble
metal component promotes the dehydrogenation function. One specific
example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists
of a ZSM-5 type zeolite, gallium and a phosphorus containing
alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction
result in rapid catalyst deactivation which is believed to be
caused by excessive carbon formation (coking) on the catalyst
surface. This coking tendency makes it necessary to frequently
perform catalyst regenerations. In addition, applicants have noted
that the prior art catalyst can be deactivated by exposure to
hydrogen at temperatures greater than 500.degree. C. Minimizing the
deactivation caused by this hydrogen exposure is a particular
object of this invention.
Applicants' catalyst contains a zeolite, a gallium component and an
aluminum phosphate binder, but is characterized in that it is
tolerant to hydrogen exposure at temperatures greater than
500.degree. C. The ability of the catalyst of this invention to
withstand extended exposure to hydrogen without significant loss of
activity is achieved by treating the catalyst with an aqueous
solution of a weakly acidic ammonium salt or a dilute acid
solution. This treatment removes some aluminum and phosphorus (and
small amounts of silicon) from the catalyst as evidenced by
analysis of the wash water. It is believed that this treatment
removes an aluminum/phosphorus species which has deleterious
effects on the catalyst when exposed to hydrogen at high
temperatures. Since the catalyst is exposed to such conditions
during normal operation, the ability to remove such a deleterious
species results in the unexpected result of increased catalyst
life.
SUMMARY OF THE INVENTION
As stated, the instant invention relates to a catalyst, a process
for preparing the catalyst and a process for using the catalyst.
Thus, one embodiment of the invention is a catalyst for converting
C.sub.2 to C.sub.6 aliphatic hydrocarbons to aromatics comprising a
zeolite having a Si:Al ratio greater than about 10 and a pore
diameter of about 5-6 .ANG., a gallium component and an aluminum
phosphate binder, the catalyst characterized in that it is tolerant
to exposure to hydrogen at a temperature of about 500.degree. to
about 700.degree. C.
Another embodiment of the invention is a process for preparing a
catalyst for converting C.sub.2 to C.sub.6 aliphatic hydrocarbons
to aromatics comprising: a) forming particles from a mixture of a
zeolite and an aluminum phosphate binder; b) calcining said
particles at a temperature of about 500.degree. to about
700.degree. C. and for a time of about 1 to about 15 hours; c)
impregnating the calcined particles with a gallium salt; d) heating
the gallium containing particles in hydrogen at a temperature of
about 500.degree. to about 700.degree. C. for a time of about 1 to
about 15 hours, followed by heating in an air/steam mixture at a
temperature of about 400.degree. to about 700.degree. C. for a time
of about 1 to about 10 hours; e) treating the particles of step (d)
with an aqueous solution of a weakly acidic ammonium salt or a
dilute acid solution at a temperature of about 50.degree. to about
100.degree. C. for a time of about 1 to about 48 hours; and f)
calcining the particles at a temperature of about 500.degree. to
about 700.degree. C. for a time of about 1 to about 15 hours,
thereby providing said catalyst.
Yet another embodiment of the invention is a process for converting
C.sub.2 to C.sub.6 aliphatic hydrocarbons to aromatic compounds
comprising contacting a feed stream containing C.sub.2 to C.sub.6
aliphatic hydrocarbons with a catalyst at dehydrocyclodimerization
conditions to provide aromatic compounds, the catalyst comprising a
zeolite having a Si:Al ratio greater than about 10 and a pore
diameter of about 5-6 .ANG., a gallium component and an aluminum
phosphate binder, the catalyst characterized in that it is tolerant
to exposure to hydrogen at a temperature of about 500.degree. to
about 700.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
As stated, this invention relates to a catalyst, a process for
preparing the catalyst and a process for using the catalyst. The
catalyst of the present invention comprises a zeolite component, a
binder component, and a gallium metal component. The zeolites which
may be used are any of those which have a Si:Al ratio greater than
about 10 and preferably greater than 20 and a pore diameter of
about 5 to 6 Angstroms. Specific examples of zeolites which can be
used are the ZSM family of zeolites. Included among this ZSM family
are ZSM-5, ZSM-8, ZSM-11, ZSM-12 and ZSM-35.
The preparation of these ZSM-type zeolites is well known in the art
and generally are prepared by crystallizing a mixture containing an
alumina source, a silica source, an alkali metal source, water and
a tetraalkyl ammonium compound or its precursor. The amount of
zeolite present in the catalyst can vary considerably but usually
is present in an amount from about 30 to about 90 weight percent
and preferably from about 50 to about 70 weight percent of the
catalyst.
A second component of the catalyst of this invention is a
phosphorus containing alumina (hereinafter referred to as aluminum
phosphate) component. The phosphorus may be incorporated with the
alumina in any acceptable manner known in the art. One preferred
method of preparing this aluminum phosphate is that described in
U.S. Pat. No. 4,629,717 which is incorporated by reference. The
technique described in the '717 patent involves the gellation of a
hydrosol of alumina which contains a phosphorus compound using the
well-known oil drop method. Generally this technique involves
preparing a hydrosol by digesting aluminum in aqueous hydrochloric
acid at reflux temperatures of about 80.degree. to 105.degree. C.
The ratio of aluminum to chloride in the sol ranges from about
0.7:1 to about 1.5:1 weight ratio. A phosphorus compound is now
added to the sol. Preferred phosphorus compounds are phosphoric
acid, phosphorous acid and ammonium phosphate. The relative amount
of phosphorus and aluminum expressed in molar ratios ranges from
about 1:1 to 1:100 on an elemental basis.
The resulting aluminum phosphate hydrosol mixture is now gelled.
One method of gelling this mixture involves combining a gelling
agent with the mixture and then dispersing the resultant combined
mixture into an oil bath or tower which has been heated to elevated
temperatures such that gellation occurs with the formation of
spheroidal particles. The gelling agents which may be used in this
process are hexamethylene tetraamine, urea or mixtures thereof. The
gelling agents release ammonia at the elevated temperatures which
sets or converts the hydrosol spheres into hydrogel spheres. The
spheres are then continuously withdrawn from the oil bath and
typically subjected to specific aging and drying treatments in oil
and in ammoniacal solution to further improve their physical
characteristics. The resulting aged and gelled particles are then
washed and dried at a relatively low temperature of about
93.degree. C. to about 149.degree. C. (200.degree.-300.degree. F.)
and subjected to a calcination procedure at a temperature of about
450.degree. C. to about 703.degree. C. (850.degree.-1300.degree.
F.) for a period of about 1 to about 20 hours. The amount of
phosphorus containing alumina component present (as the oxide) in
the catalyst can range from about 10 to about 70 weight percent and
preferably from about 30 to about 50 weight percent.
The zeolite and aluminum phosphate binder are mixed and formed into
particles by means well known in the art such as gellation,
pilling, nodulizing, marumerizing, spray drying, extrusion or any
combination of these techniques. A preferred method of preparing
the zeolite/aluminum phosphate support involves adding the zeolite
either to an alumina sol or a phosphorus compound, forming a
mixture of the alumina sol/zeolite/phosphorus compound which is now
formed into particles by employing the oil drop method described
above. The particles are calcined as described above to give a
support.
Another necessary component of the instant catalyst is a gallium
component. The gallium component may be deposited onto the support
in any suitable manner known to the art which results in a uniform
dispersion of the gallium. Usually the gallium is deposited onto
the support by impregnating the support with a salt of the gallium
metal. The particles are impregnated with a gallium salt selected
from the group consisting of gallium nitrate, gallium chloride,
gallium bromide, gallium hydroxide, gallium acetate, etc. The
amount of gallium which is deposited onto the support varies from
about 0.1 to about 5 weight percent of the finished catalyst
expressed as the metal.
The gallium compound may be impregnated onto the support particles
by any technique well known in the art such as dipping the catalyst
into a solution of the metal compound or spraying the solution onto
the support. One preferred method of preparation involves the use
of a steam jacketed rotary dryer. The support particles are
immersed in the impregnating solution contained in the dryer and
the support particles are tumbled therein by the rotating motion of
the dryer. Evaporation of the solution in contact with the tumbling
support is expedited by applying steam to the dryer jacket. After
the particles are completely dry, they are heated under a hydrogen
atmosphere at a temperature of about 500.degree. to about
700.degree. C. for a time of about 1 to about 15 hours. Although a
pure hydrogen atmosphere is preferred to reduce and disperse the
gallium, the hydrogen may be diluted with nitrogen. Alternatively,
it is envisioned that the reduction and dispersion can be done in
situ in the actual reactor vessel used for dehydrocyclodimerization
by using either pure hydrogen or a mixture of hydrogen and
hydrocarbons. Next the hydrogen treated particles are heated in air
and steam at a temperature of about 400.degree. to about
700.degree. C. for a time of about 1 to about 10 hours. The amount
of steam present in the air varies from about 1 to about 40
percent.
These catalyst particles which now contain well dispersed gallium
present as gallium oxide are treated with an aqueous solution of a
weakly acidic ammonium salt or a dilute acid solution. The ammonium
salts which can be used include ammonium chloride, ammonium acetate
and ammonium nitrate. The concentration of these salts can vary
from about 0.1 to about 5 molar. The acids which can be used
include hydrochloric, acetic, nitric and sulfuric acid. Although
concentrated acids could be used, they would degrade the zeolite
and the integrity of the particles as well as removing the
undesirable aluminum phosphorus species. Thus, it is desirable to
use dilute acids which have a molarity from about 0.1 to about 5
moles/liter. Of these treatment solutions, it is preferred to use
an ammonium nitrate solution.
The treating solution is contacted with the calcined catalyst
particles at a temperature of about 50.degree. to about 100.degree.
C. for a time of about 1 to about 48 hours. After this treatment,
the particles are separated from the aqueous solution, dried and
calcined at a temperature of about 500.degree. to about 700.degree.
C. for a time of about 1 to about 15 hours, thereby providing the
catalyst of the instant invention.
The purpose of treating the support with one of the solutions
described above is to remove materials which cause the catalyst to
deactivate when it is exposed to hydrogen (hydrogen is produced
during the dehydrocyclodimerization process) at temperatures above
500.degree. C. and specifically temperatures between 500.degree.
and 700.degree. C. The exact nature of the species which is removed
by this treatment step is not known. Without wishing to be bound by
a particular theory, it is postulated that the deleterious species
which is removed is an aluminum/phosphorus species. This hypothesis
is based on the analysis of the wash water after the catalyst has
been treated. The wash water also contains small amounts of silicon
indicating that the deleterious species may also contain
silicon.
Although the exact nature of the species which is removed is not
known, it is observed that treating a catalyst which contains an
aluminum phosphate binder with one of the solutions described above
renders the catalyst tolerant to hydrogen exposure at temperature
above 500.degree. C. By tolerant is meant that the catalyst can
operate for a much longer period of time without significant loss
in activity. In the instant case, the treated catalyst has a
lifetime which is at least 6 times longer than a catalyst which has
not been treated according to this invention.
The dehydrocyclodimerization conditions which will be employed for
use with the catalyst of the present invention will, of course,
vary depending on such factors as feedstock composition and desired
conversion. A desired range of conditions for the
dehydrocyclodimerization of C.sub.2 -C.sub.6 aliphatic hydrocarbons
to aromatics include a temperature from about 350.degree. C. to
about 650.degree. C., a pressure from about 1 to about 20
atmospheres, and a liquid hourly space velocity from about 0.2 to
about 5 hr.sup.-1. The preferred process conditions are a
temperature in the range from about 400.degree. to about
550.degree. C., a pressure in or about the range from 2 to 10
atmospheres and a liquid hourly space velocity of between 0.5 to
2.0 hr.sup.-1. It is understood that, as the average carbon number
of the feed increases, a temperature in the lower end of the
temperature range is required for optimum performance and
conversely, as the average carbon number of the feed decreases, the
higher the required temperature.
The feed stream to the dehydrocyclodimerization process is defined
herein as all streams introduced into the dehydrocyclodimerization
reaction zone. Included in the feed stream is the C.sub.2 -C.sub.6
aliphatic hydrocarbon. By C.sub.2 -C.sub.6 aliphatic hydrocarbons
is meant one or more open, straight or branched chain isomers
having from two to six carbon atoms per molecule. Furthermore, the
hydrocarbons in the feedstock may be saturated or unsaturated.
Preferably, the hydrocarbons C.sub.3 and/or C.sub.4 are selected
from isobutane, normal butane, isobutene, normal butene, propane
and propylene. Diluents may also be included in the feed stream.
Examples of such diluents include hydrogen, nitrogen, helium,
argon, neon.
According to the present invention, the feed stream is contacted
with the instant catalyst in a dehydrocyclodimerization reaction
zone maintained at dehydrocyclodimerization conditions. This
contacting may be accomplished by using the catalyst in a fixed bed
system, a moving bed system, a fluidized bed system, or in a batch
type operation; however, in view of the danger of attrition losses
of the valuable catalyst and of the well-known operational
advantages, it is preferred to use either a fixed bed system or a
dense-phase moving bed system such as shown in U.S. Pat. No.
3,725,249.
In a fixed bed system or a dense-phase moving bed the feed stream
is preheated by any suitable heating means to the desired reaction
temperature and then passed into a dehydrocyclodimerization zone
containing a bed of the instant catalyst. It is, of course,
understood that the dehydrocyclodimerization zone may be one or
more separate reactors with suitable means therebetween to assure
that the desired conversion temperature is maintained at the
entrance to each reactor. It is also important to note that the
reactants may be contacted with the catalyst bed in either upward,
downward, or radial flow fashion with the latter being preferred.
In addition, the reactants are in the vapor phase when they contact
the catalyst. The dehydrocyclodimerization system then preferably
comprises a dehydrocyclodimerization zone containing one or more
fixed or dense-phase moving beds of the instant catalyst. In a
multiple bed system, it is, of course, within the scope of the
present invention to use the present catalyst in less than all of
the beds with another dehydrocyclodimerization or similarly
behaving catalyst being used in the remainder of the beds. This
dehydrocyclodimerization zone may be one or more separate reactors
with suitable heating means therebetween to compensate for any heat
loss encountered in each catalyst bed. Specific to the dense-phase
moving bed system, it is commmon practice to remove catalyst from
the bottom of the reaction zone, regenerate it by conventional
means known to the art, and then return it to the top of the
reaction zone.
The following examples are presented in illustration of this
invention and are not intended as undue limitations on the
generally broad scope of the invention as set out in the appended
claims.
EXAMPLE 1
This example describes the preparation of a
dehydrocyclodimerization catalyst according to the prior art. A
first solution was prepared by adding phosphoric acid to an aqueous
solution of hexamethylenetatraamine (HMT) in an amount to yield a
phosphorus content of the finished catalyst equal to about 11
weight percent. A second solution was prepared by adding a ZSM-5
type zeolite to enough alumina sol, prepared by digesting metallic
aluminum in hydrochloric acid, to yield a zeolite content in the
finished catalyst equal to about 67 weight percent. These two
solutions were commingled to achieve a homogeneous admixture of
HMT, phosphorus, alumina sol, and zeolite. This admixture was
dispersed as droplets into an oil bath maintained at about
93.degree. C. The droplets remained in the oil bath until they set
and formed hydrogel spheres. These spheres were removed from the
oil bath, water washed, air dried, and calcined at a temperature of
about 482.degree. C. A solution of gallium nitrate was utilized to
impregnate the spheres to achieve a gallium content on the finished
catalyst equal to about 1 weight percent. After impregnation, the
spheres were dried, then heated in pure hydrogen at 580.degree. C.
for 6 hours. The spheres were finally calcined, in the presence of
steam, at a temperature of about 649.degree. C. This catalyst was
identified as catalyst A.
EXAMPLE 2
This example describes the preparation of a
dehydrocyclodimerization catalyst according to the invention. About
100 cc of Catalyst A were added to a round bottom flask equipped
with a condenser and containing 500 mL of a 2M ammonium nitrate
aqueous solution. This mixture was heated to reflux by means of an
oil bath and refluxed for 24 hours. Upon cooling the mixture was
filtered and the spheres washed five times with 100 mL of deionized
water. The washed spheres were dried at 150.degree. C. for 3 hours
and then calcined at 540.degree. C. for 2 hours. This catalyst was
identified as catalyst B.
The wash water from the ammonium nitrate treatment was analyzed to
determine what elements were present. The analysis of the wash
water and catalyst B are presented in Table A.
TABLE A ______________________________________ Catalyst B Wash
Water Element (wt. %) (wt. %)
______________________________________ Si 23.33 0.05 Al 13.50 0.188
P 10.96 0.108 Ga 1.02 0.00003
______________________________________
As Table A shows, the species which are removed by the ammonium
nitrate wash are composed primarily of aluminum and phosphorus.
EXAMPLE 3
The following test procedure was used to evaluate the
dehydrocyclodimerization activity of catalysts. A feedstock of
propane was flowed through a reactor containing the catalyst to be
tested. The propane was flowed through the reactor at a liquid
hourly space velocity of 0.8 hr.sup.-1 under a pressure of 1
atmosphere and at a reactor inlet temperature of 540.degree. C. The
conversion of propane to aromatics was calculated at various times
during the testing.
Catalysts A and B were accelerated aged by treating the catalysts
with a hydrogen/methane gas feed at 1 atmosphere and 565.degree. C.
for a period of 100 hours. The function of the methane is to act as
a diluent. After this hydrogen treatment, the catalysts were
oxidized in air at 565.degree. C. and then tested as described
above. This hydrogen treatment is believed to be analogous to about
one month of on stream operation. The results of the testing are
presented in Table B.
TABLE B ______________________________________ Effect of NH.sub.4
NO.sub.3 Treatment on the Activity of Aged Catalysts Conversion (%)
Catalyst A* Catalyst B* Time (Hrs) (No Treatment) (Treated)
______________________________________ 11 69.9 78.3 23 60.4 68.8 35
54.8 60.1 47 N/A N/A ______________________________________ *Aged
for 100 hours with H.sub.2 /CH.sub.4 at 565.degree. C.
The results presented in Table B show that a catalyst that has been
treated with ammonium nitrate has a much higher conversion than one
that has not been treated. Therefore, Catalyst B is much more
tolerant to hydrogen exposure than Catalyst A.
* * * * *